Low voltage integrated circuitry has steadily improved over the years. Presently, low voltage integrated circuit devices commonly operate in the two to three volt range. Low voltage operation provides, among other benefits, low power consumption. Thus, in battery operated devices, such as portable telephones, pagers, lap-top computers and the like, low voltage integrated circuitry allows the devices to operate proportionally longer than devices operating at higher voltages.
Low voltage operation, while providing many benefits, causes problems with respect to some of the circuitry contained in the integrated circuit. Field effect transistors, which are commonly used for switching, require minimum gating voltages to operate in favorable ranges. One such device that has minimum gating requirements is called a T-gate. T-gates are commonly used as series elements in MOS implementations. A typical T-gate comprises the parallel combination of a P-channel MOS device and an N-channel MOS device, connected such that the drains of the devices connect and the sources of the devices connect. The T-gate is turned-on to pass current when the gate of the P-channel device is held at a low voltage while the gate of the N-channel FET is held at a high voltage.
As illustrated in FIG. 1, however, the series impedance of a T-gate depends upon the voltages between the drains and the sources of both FET transistors and upon the voltages applied to their gates. As shown, when the gate voltage applied to the gate of the N-channel MOS device of a T-gate switch is relatively low, the series impedance of the T-gate can be large. The relationship between gate voltage and series impedance relates to the manner in which the respective MOS devices operate. With smaller drain-to-source voltages, the N-channel MOS device has a much lower series impedance. However, at larger drain-to-source voltage levels, the P-channel MOS device has a much lower series impedance. Thus, the combination of the two MOS devices has a low series impedance at very low drain-to-source voltages, rises to a peak series impedance, and then has a lower series impedance at higher drain-to-source voltages. Also, as is shown, the series impedance of the T-gate reduces dramatically with an increased gate voltage applied to the gate voltage on the N-channel device. As one skilled in the art will readily appreciate a reduction in gate voltage on the P-channel device will also dramatically reduce the series impedance of the T-gate.
Thus, in order to reduce the series impedance of the T-gate switches, it is desirable to provide sufficient gate voltages to the T-switches. However, in low voltage, low power applications, the source voltage V.sub.DD is typically very small. Resultantly, circuits have been developed to amplify the source voltage V.sub.DD for switching applications.
One such device used to amplify voltages is called a charge pump. Charge pumps, supplied by a source voltage V.sub.DD, operate in a two-stage switched mode to provide an amplified voltage at an output. In a first phase of the charge pump's operation, a capacitor is charged with a source voltage to the level of the source voltage. Then, on a second phase of the charge pump's operation, the circuit is switched such that the source voltage and capacitor are connected in series to an output so as to create an amplified voltage at the output. The charge pump is capable of providing as much as twice the source voltage V.sub.DD at the output. Charge pumps may be employed as power supplies by driving output capacitors, as clock supplies by grounding the output, and other devices known in the art.
While the charge pump provides an increased voltage to circuits that require increased voltages, the supplied voltage level varies depending on load variations and battery supply variations. When the load is relatively large and the battery supply is relatively low, the charge pump supplied voltage will be low and thus exhibiting the same problems as mentioned above. Contrastly, when the load is relatively small and the battery supply is relatively large, the charge pump supplied voltage may be too large, which can destroy oxide layers and otherwise reduce the lifecycle of the integrated circuit elements. Thus, regulating the output voltage of the charge pumps is important.
A prior art method of regulating the output voltage of a charge pump included stacking diodes at the output of the charge pump to prevent the output voltage from exceeding a maximum voltage. When the output voltage of the charge pump reached the "turn-on" voltage of the diode stack, current flowed through the diode stack to ground. In low power applications, any current drain is undesirable. Therefore, while this technique prevents overvoltage conditions, it has the very undesirable side effect of increased power consumption and does not regulate the charge pump voltage for undervoltage conditions.
Thus, a need exists for a regulated charge pump that does not shunt charge to ground and that automatically reacts to changing load conditions so as to protect against over voltage conditions and under voltage conditions.